The present invention relates to a laser machining apparatus which uses a laser beam to, for example, trim thin film resistors and devices and, more particularly, to a beam positioner for a laser machining apparatus for moving a laser light beam.
While laser machining is generally performed by moving either a laser beam or a device to be machined, beam moving systems are predominant today due to inherently high throughput attainable therewith. A typical beam positioner is implemented with the combination of a galvanometer type optical scanner and a so called f.times..theta. objective lens and is particularly useful for high-speed operation. The foregoing type of system is described by Alan Cable in an article entitled "Solving the High Speed Laser Trimming Puzzle", published by Hybrid Circuit Technology, September 1984, pp. 57-61.
An optical scanner ordinarily consists of an X-axis scanner and a Y-axis scanner adapted to deflect a laser beam in the orthogonal X- and Y-axis directions, respectively, and respective drivers associated with each of the scanners. The scanners in most cases are implemented with galvanometers. The drivers respectively drive the X- and Y-axis scanners responsive to X-axis and Y-axis deflection control signals which are outputted by a control circuit of a laser machining apparatus. In this way a respective mirror mounted on each scanner is rotated by predetermined angles. A light beam is reflected by the X-axis and Y-axis mirrors and then passed through the f.times..theta. lens to be focused onto a workpiece. A f.times..theta. lens is an objective lens having such a characteristic that, assuming the focal length is f and the angle between the optical axis and an incident light beam is .theta., the focussing point of the beam lies in the focal plane of the lens and is spaced f.times..theta. from the focus of the lens. The focal point of the beam on a work surface is proportional to the deflection angle of the mirror. A galvanometer for use with this scanner is applied to various types of apparatus, especially a laser trimmer, because the mirror develops only low moments of inertia during small angular movements thereby allowing a laser beam to move at high speed.
However, a problem with the above-described prior art beam positioner is that because the light beam is sequentially deflected by two optical scanners and then passed through an f.times..theta. lens, its locus on a work surface is somewhat deviated from an ideal one, resulting in distortion which is usually referred to as scan distortion. Such scan distortion prevents a laser trimmer from accurately trimming microscopic patterns.